402 University of California Publications in Geology [Vol. 8 



The North Side of Mount Diablo 

 8TRATIGBAPHY AND LITHOLOGY 



The San Pablo Group on the north side of Mount Diablo has a 

 maximum thickness of about six hundred feet and a minimum thick- 

 ness of about four hundred feet. 



The deposits, especially in the lower part of the section, may be 

 classed as distinctly those of a strand line, being largely composed of 

 coarse conglomerates and cross-bedded sandstones. East of Kirker 

 Creek, there is a well-marked irregular contact within and about three 

 hundred feet above the base of the group, and at several localities the 

 trunks of silicified trees were found standing at right angles to the 

 contact. No difference in dip and strike was obtained between the 

 beds above and below the unconformity. 



The San Pablo above the unconformity in this section is formed 

 largely of layers of bright blue sandstone and pearl-gray, tuffaceous 

 shale. Near the top several small lenses of conglomerate were observed. 

 There is considerable tuffaceous material throughout the group, and 

 in the upper part of the section thin layers of white tuff are quite 

 common. 



In this area the group unconformably overlies the Monterey Group 

 and is in turn overlain unconformably by the Pinole Tuft. The un- 

 conformity between the Monterey and San Pablo in this locality is of 

 such a character as would seem to represent a fairly notable hiatus. 

 Briefly, the evidence is as follows : 1. At certain localities there is a 

 difference in strike and dip between the two groups. 2. An irregular 

 contact between the two was found at several places. 3. The basal 

 beds of the group rest on different members of the Monterey as one 

 follows the line of strike. 4. Borers of the pholadid type were found 

 in the Monterey rock along the contact for a distance of over three 

 miles. 5. Well-rounded pebbles lithologically identical with the 

 tuffaceous sandstone and shale of the Monterey below, are found in 

 the basal conglomerate. 



FAUNA 



The fauna obtained from this section of the San Pablo Group 

 was described by the writer 22 in a former paper. A few changes have 

 been made in the faunal list and a few species have been added to it. 



-- Clark, Bruce L., op. cit. 



